Oxygen-derived free radical stress activates nonselective cation current in guinea pig ventricular myocytes. Role of sulfhydryl groups.

Oxygen-derived free radicals (O-Rs) cause alterations in cardiac electrical activity, including sustained depolarization, which may contribute to arrhythmic activity in reperfusion after ischemia. The ionic current(s) and cellular mechanism(s) underlying the sustained depolarization are not well defined. We used the whole-cell variant of the patch-clamp technique to study sustained depolarization in guinea pig ventricular myocytes during the extracellular application of O-Rs (generating system: dihydroxyfumaric acid, 3 to 6 mmol/L; FeCl3/ADP, 0.05:0.5 mmol/L). Myocytes superfused with O-Rs (pipette EGTA, 0.1 mmol/L) showed (1) sustained depolarization to between -40 and -10 mV, (2) oscillations in membrane potential, and (3) triggered activity. The depolarization resulted from an increase in quasi-steady state difference current reversing at approximately -18 mV, and the oscillations were due to transient inward current. The latter were inhibited with ryanodine (10 mumol/L) or high pipette EGTA (5 mmol/L), but the steady state current was unaffected. Nonselective cation current (INSC) (recorded with Cs+, Li+, and Mg2+ replacing K+, Na+, and Ca2+, respectively; 20 mmol/L tetraethylammonium chloride [TEA] and 5 mmol/L BAPTA in the pipette solution and 10 mmol/L TEA, 10 mumol/L tetrodotoxin, and 10 mumol/L nicardipine in the bath solution) was activated by O-Rs; the increase in current was unaffected by preventing changes in [Ca2+]i but was inhibited with dithiothreitol. Oxidizing agents (diamide and thimerosal) or caffeine (pipette EGTA, 0.1 mmol/L) produced a similar increase in membrane conductance. INSC activated with O-Rs, oxidizing agents, or caffeine was sensitive to SK&F 96365. O-R treatment was without effect when INSC was already activated with caffeine. The data suggest that (1) extracellular O-Rs activate a Ca(2+)-sensitive INSC in the absence of changes in [Ca2+]i, (2) oxidative modification of extracellular sulfhydryl groups may be involved, and (3) this mechanism is different from the Ca(2+)-dependent activation of INSC by intracellular O-Rs, indicating that O-Rs may alter ion channel activity by differential mechanisms, depending on the compartment, extracellular or intracellular, in which they are present.